Loud-speakers



April 16, 1957 J. B. BRENNAN 2,789,161

LOUD-SPEAKERS 3 Sheets-Sheet 1 Filed April 5, 1951 15 v INVENTOR! y 4 V ATTORNEYS.

April 16, 1957 J. B. BRENNAN LOUD-SPEAKERS 3 Sheets-Sheet 2 Filed April 5 1951 ATTORNEYS.

R m w w April 16, 1957 J. B. BRENNAN 2,789,161

' LOUD-SPEAKERS Filed April 5, 1951 3 Sheets-Sheet 3 24 I 2; 22, gnu

ATTORNEYS.

United States Patent 2,789,161 LOUD-SPEAKERS Joseph B. Brennan, Cleveland, Ohio Application April 5, 1951, Serial No. 219,451 6 Claims. (Cl. 179-1155) This invention relates to loud speakers such as are employed in radio speakers, public address systems and the like.

A general object of the invention is the provision of an improved loud speaker in which increased efliciency in the operation, and increased efiiciency in the use of materials in constructing the apparatus are obtained. Other objects of the invention are the provision of a loud speaker whereby increased fidelity of sound reproduction can be obtained as compared to present practices; the provision of a loud speaker whereby high efiiciency and intensity of sound output can be obtained while maintaining the fidelity of sound reproduction; the provision of an acoustic diaphragm and a loud speaker embodying the same whereby substantially the entire diaphragm is caused to vibrate in unison; the provision of a diaphragm and loud speaker assembly in which the ratio of power input as compared to the mass of diaphragm may be high, whereby the diaphragm is constrained to vibrate in accordance with the electric currents supplied to the loud speaker with a high degree of fidelity; and the provision of a-loud speaker embodying a diaphragm having a plurality of voice coils, and which can be manufactured economically and by simple manufacturing operations.

Further objects and advantages of my invention will become apparent from the following description of preferred :forms thereof,'reference being made to the accompanying drawings wherein Fig. l is a rear elevation of a preferred form of acoustic diaphragm embodying my invention; Fig. 2 is a section, on an enlarged scale, through the diaphragm of Fig. 1 taken along the line 2-2 of Fig. 1; Fig. 3 is a generally similar sectional view showing a loud speaker embodying the diaphragm of Figs. 1 and 2 with parts of the loud speaker housing broken away for convenience in illustration; Fig. 4 is a rear elevation, on :a reduced scale, of the loud speaker shown in Fig. 3 with parts of the housing broken away for convenience in illustration; Figs. 5 and 6 are sectional views generally similar to Fig. 2 but showing modified forms of diaphragms; Fig. 7 is a rear elevational view of a still further modification of my diaphragm; Fig. 8 is a sectional view illustrating a loud speaker embodying the diaphragm of Fig. 7 and taken on the plane indicated by line 88 of Fig. 7; Fig. 9 is a transverse sectional view of a loud speaker embodying another modification of my invention; Fig. 10 is a front view of the speaker shown in Fig. 9 on a slightly reduced scale, the line 9-9 of Fig. 10 indicating the section plane of Fig. 9; Fig. 11 is a transverse sectional view taken along a line corresponding to the lines 22 of Fig. 1 and 88 of Fig. 7 and illustrating a loud speaker embodying a still further specification of my invention; and Fig. 12 is a fragmentary section of a further modification of the invention showing how a spider may be used in positioning a loud speaker of the invention. Throughout the drawings the thicknesses of various portions of the diaphragm themselves are necessarily somewhat exaggerated.

i Briefly my invention, in its preferred forms, contem- "ice plates an acoustic diaphragm having a body portion adapted to be vibrated to generate sound, said body having, instead of the usual single, centrally located voice coil support, a plurality of voice coil supports spaced about the body portion to distribute the vibratory forces applied thereto. The body portion may be hollowor fiat, and may be of conical shape with the centers of the voice coil supports symmetrically disposed between the center of the diaphragm and the outer peripheral edge thereof. The diaphragm may be supported at its outer edge in theusual manner and may also be centered at its central portion by a spider or other convenient means which may be either unitary'with or separate from the body of the diaphragm. Each voice coil support ispro vided with a voice coil, and the diaphragm is incorporated in a loud speaker structure in which a separate field struc ture is provided for each voice coil associated with the diaphragm.

In use, the several voice coils may be connected in parallel or series depending upon the electrical characteristics of the coils and the circuit with which the device is used, and thus the voice coils vibrate in unison in response to the voice currents of the radio receiver, loud speaker or the like with which the device is employed. With this arrangement, instead of attempting to vibrate the entire diaphragm from a single coil at the apex of the diaphragm as is the conventional practice, the diaphragm is vibrated by like forces applied at spaced points around the diaphragm with the result that the entire diaphragm vibrates in phase and much more nearly as a unit than is the case with a conventional diaphragm embodying only a single voice coil. Greater volumeof sound output per unit of electrical input andhigher fidelity of reproduction, can be obtained with such diaphragms than is possible with a conventional diaphragm of the same size.

Referring now to Figs. 1 and 2 of the drawings, one preferred form of acoustic diaphragm embodying my invention is indicated in general at 10. This comprises a peripheral supporting flange 11 joined by the corrugated flexible portion 12 to the hollow conical body portion 14 of the diaphragm. The diaphragm is provided with a plurality of tubular necks 15 projecting therefrom which are adapted to support the usual voice coils 16. Usually .these necks 16 are glued or cemented to integral necks between about .010" and A" long on the diaphragm. In the embodiment shown there are six tubular voice coil supporting necks 15, each with its associated voice coil 16. The voice coils may be mounted on the supports in any convenient manner, and may be provided with the usual lead wires (not shown). Alternatively, the construction disclosed in the Moynihan Patent No. 2,328,836 may be utilized and the necessity for lead wires eliminated.

The voice coil supports are preferably spaced uniformly inwardly from the edge of the diaphragm and are also spaced uniformly in circumferential directions. Thus with a circular diaphragm as shown, the axes, of the several voice coil supports 15 are parallel to each other and to the axis of the body portion and are evenly spaced around a circle which is concentric with the diaphragm itself. However, those skilled in the art will understand .that different spacing arrangements may be employed to suit difierent purposes and that the axes of the voice coil supports need not lie on a circle, particularly in instances where the diaphragms are of elliptical, rectangularor other non-circular shape; in such cases the axes prefer.

ably lie on a line which is similar in shape to the periphery of the body portion, i. e., the juncture of the body chosen depending upon the size of the diaphragm, the power output desired and other design considerations.

In the arrangement shown in Figs. 1 and 2, the center lines of the axes of the voice coil supporting members 15 lie on a circle which divides the projected area of the conical diaphragm into two substantially equal parts. That circle is indicated in broken line in Fig. 1' at 17, and the arrangement is such that the projected area of the body {of the diaphragm within the circle 17 is substantially equal to the projected area of the body of the dia- ;phragm between the circle 17 and the juncture of the body of the diaphragm within the corrugated portlon .12. This is ?one method of distribution of the voice coil sup- ;porting members with respect to the body pf the d 1azphragm. which may be employed to obtain substantially equal vibratory effects throughout the diaphragm, 1. e., to produce a diaphragm in which all parts of the diaphragm vibrate in approximately identical fashion at any given instant. Obviously, it is difiicult, if not impossible, :to determine the frequency and amplitude of vibration of all parts of a diaphragm at a given instant, but it appears probable that the distribution of the :voice coil supporting members are described above which results in a corresponding distribution of the vibratory forces applied to the "diaphragm 'when it is installed in a loud speaker gives -a much better approximation of this desired result "thanhas been obtainable heretofore.

' The diaphragm is preferably integrally formed of fibrous, felted material. The diaphragms may be made by depositing fibrous material from a suspension in liquid upon a. perforate form in the usual manner, or m'ay be produced by spraying fibrous material and ;a binder upon 'a-form preferably in the manner'described i'niny copending application, Ser. No. 422,388, filed December '10, 1941. "Depending upon thecharacteristics desired, the felted material may be subject-to further operations 'afterit is initially deposited, or it maybe used after 'mer'ely drying. A preferred method of drying or curing is disclosed in my copending applicationserlNo. 615,720, filed "of even date herewith. As examples of other'treatments the deposited material may be s1ibjected 'to 'a'pressing operation,may'-be cured "by heat, an'd/or .m'ay be impregnated with va'riousstifieningandstrength- 0 spokes 34.

e'nin'g materials such as various lacquers,' resins, biuders and the' like. The thickness of various "portions of the diaphragm may be varied to provide difierent character- 'istics .of flexibility; thus the "edge portions of the dia- "phragm may be constructed as described 'in my' Patent No. "2,302,178 in order to provide increased flexibility. Ifde'fsired, and as fdescribed'below, fth'e bodydf the diafphrag'm"rnay'be provided with reinforcing portions of increased'thickness and stifinessjas describedin my co- "pendin'g application Ser. No. 422,388 aforesaid. It is :believed that no detailed description of various materials for and metho'ds of, making the diaphragms is necessary for the -purposesfof the "present disclosure inasmuch as appropriate mate'rials and methods are I known to: those skilled in the art'and/or are 'disclosed in.- my patent and -japplications aforesaid.

Diaphragms embodying-my invention lend themselves readily to incorporation in loud speakers of simple and elficient design. An example of .su ch aloud speaker embodying the diaphragm 10 of Figs. 1 and .2 is. illus- {traded in Figs. *3an'd 4. "The device conaprises .asupir n amer i i suppo t n pl t 20 t iiwhig ar plurality h of field structures, e'ach in dicated in general at 21, :are secured; the field structures f 'beingspaced to correspond to the spacingof the voice coil supports-15 "and coils 16. As shown-particularly in Fig. 3, each field "structure comprises a hollow cup-shaped member 22 provided with an o'pen end to 'whichl a plate 23 is secured, -for example byscrews '24, and with an. aperture-.25 at else end oppositethe plate-23. iA pole piece 2'6 is pressed intman openi-ng in the plate-#23 or otherwise suitably.-se-

4 cured thereto and projects through the opening 25. The pole piece is accurately centered With respect to the opening 25, providing an annular air gap within which the voice coil 16 and support 15 are disposed.

In the form shown, the requisite magnetic field is set up by means of a coil 27 surrounding pole piece 26 and disposed within the member 22 in the usual manner, but those skilled in the art will understand that permanent magnet fields may *be employed. The flow of voice currents through the voice coil will cause the coil to vibrate in the magnetic field set .up by the coil .in' accordance with the direction, frequency and amplitude of the voice currents as is well known. The field structures '21 are all preferably identical with each vother, are energized in the same manner, and each coacts With its associated voice coil 16 in the same way. The voice coils are preferably identical and are connected in parallel or in series in the output circuit of the radio receiver, public address .syste m or the like with which the speaker is used. Therefore, substantially identical forces are applied at any given instant to all of the voice coils associated with the diaphragm. V

The diaphragm may be centered with respect .to the plate -20 by a spider 2 9 .connectedto .thelindented portion '30 of the plate and cemented into the outer surface of the diaphragm. .As shown particularly in Fig. 4, the {spider may comprise a rim portion 131 .secured to the diaphragm, a central :hub portion 32 secured to the .plate as-by the belt or screw 33 and a-plurality of flexiblespiral The plate 20 supports the several field structures in proper relationship to coact with the coils of the diaphragm, :the'plate being provided with an annular indentation 36 and having holes'thereincorresponding to .the position of the axes .of the .voice coils. Screws 37 threaded into the pole pieces 26 securezthe respective-field structures in the'desired positions,.the,holes .in the plate ;20 preferably beingsomewhatlarger than the .shanks of .the screws to permit of a limited amount .of adjustment .to :enable the pole pieces and members -22 to be .accurately centered .with respect ,to thelvoice coils and;.supports. :Itwillbe-noted-.thattthe.voice.coil supports 15 fare of .s ufiicienhlength sothat the .voice calls 16 are. positioned -;within the :air .gap and .at :ihfi same time there {is tsufficient clearance between athe .inner portion 0f the :member-22 and the surface.ofthediaphragmas indicated :at 38. 'Thenecessary length, ofcourse, :depends upon .the .diameterof .the .member :22 .and the shape ,.of :the diaphragm.

;In order 'to complete .the frame structure :for .the loud :speaker. mechanism :andsto. support the. outer rim :or flange .11 of :the diaphragm, '-I preferably .provide an annular member 40 having-ian inwardly extending5fiange41 which is secured as. byself threadingiscrews -42 :to :the outer isurface'of-the plate 20. :The -openings in the flange :41 which receive the :screws 42 maytbe made oversiie to permit adjustments to compensate for inaccuracies in "manufacture and to insure accurate emdunting of the supporting flange 11 ;an d :of the-rentire {diaphragm with respect to'the --several-field structures. At its opposite end the member 40 is flanged I outwardly as at '44, providing an annular surface to receive the supporting-flange 11 of the-diaphragm 10. 'The 'fiange F11 may-besecured in 'position -b y-a clamping ring 45 which is;he'ld-= ag'ainst the-{flange by the inwardly e xtendingportion .46 of 5 the ..:-e e "l s fidi tefi ne "ti 'fe rt e s 45 b limped or spun into engagement with the ringij i s fiafter -the parts have been1assembled and accurately adjusted to proper position.

and'a corrugated 52.- :Heresthe body portion section, for example, a parabolic shape may be employed. The diaphragm is also modified slightly in that the voice coil supporting members-55 and voice coils 56 are disposed substantially midway between the center of the diaphragm and the outer peripheral edge of the body portion. This arrangement places the voice coil supports 55 on a flatter portion of the curve of the body of the diaphragm and makes it possible to employ shorter voice coil supports and still obtain the requisite clearance between the field structures and the diaphragms. Obviously the positioning of the voice coil supports described in connection with Figs. 1 and 2 may also be employed in diaphragms of this type, or if desired the spacing shown in Fig. 5 may be employed with conical diaphragms such as shown in Figs. 1 and 2.

Fig. 6 illustrates a further modification wherein the diaphragm indicated in general at 60 has a supporting flange 61 and a flexible corrugated portion 62 but in which the body 64 of the diaphragm is shaped so that the voice coil supports 65, carrying voice coils 66, are disposed on an annular flat surface 67 interposed between a conical portion 68 and a curved portion 69 of the body. This arrangement also makes possible the utilization of shorter voice coil supports without interference between the diaphragm and the several field structures. In this modification, the voice coil supports are again positioned about midway between the center of the diaphragm and the edge of the body portion where the body portion joins the corrugated portion. Here again, the spacing described in connection with Figs. 1 and 2 may be employed. Instead of utilizing a curved central portion as shown, the central portion may be conical as shown in Figs. 1 and 2. Also, if desired, a curved central portion such as the portion 69 of Fig. 6 could be employed in conjunction with the diaphragm of Figs. 1 and 2.

The diaphragms of Figs. 5 and 6 may be incorporated in loud speakers of designs similar to that shown in Figs. 3 and 4 except for the positioning of the several field structures which obviously must be spaced to correspond to the spacing of the voice coil supports. Also, it may be necessary to reduce the diameter of the field structures somewhat for the same size of diaphragm, for with the field structures disposed closer to the center of the diaphragm they are necessarily spaced closer to each other.

In Figs. 7 and 8 I have illustrated a diaphragm and loud speaker adapted for particularly heavy duty. As before, the diaphragm indicated generally at 70 is provided with a flange portion 71 and a flexible corrugated portion 72. The body 74 of the diaphragm is constructed as described in my copending application Serial No. 422,388 aforesaid with reinforcing areas to increase the strength and rigidity of the diaphragm. The diaphragm may be of conical form, and the voice coil supporting members 75 carrying voice coils 76 are preferably positioned as described in connection with Figs. 1 and 2. Here, however, there is an additional voice coil support 77 carrying a voice coil 78 and disposed in the center of the diaphragm, and the diaphragm is reinforced by thickened annular portions 79 surrounding each voice coil support 75, a central thickened annular portion 80 surrounding the voice coil 77. Also, I may provide radial thickened reinforcing portions 81 extending between the central reinforcing portion 80 and the reinforcing portions 79, arcuate reinforcing portions 82 extending beeen the reinforcing portions 79, and/or radial reinforcements 83 extending outwardly from the annular reinforcements 79 to the juncture of the body portion 74 with the corrugated portion 72 of the diaphragm.

This arrangement produces an exceptionally rigid and powerful diaphragm which, however, may be made of reasonably light weight. The reinforcing portions insure that the entire diaphragm will vibrate substantially as a unit atgall times even under high input of power.. The thickness and the width of the reinforcing portions may be varied in accordance with requirements; however, I prefer that the reinforcing portions-be at least several times as thick as the remainder of the diaphragm, and I preferably produce them by the method described in my aforesaid application Ser. No. 422,388.

As shown in Fig. 8, diaphragms of this type may be incorporated in loud speakers generally similar to the structures shown in Figs. 3 and 4, and in Fig. 8, the same reference characters that were used in Figs. 3 and 4 have been applied to those parts which are identical with those previously described. However, the plate 20a of Fig. 8 is provided with a recessed portion as' at 85 in order to receive the additional field structure 86 required for the central voice coil 78. This structure may be identical with the field structures 21 heretofore described and may be secured to the plate 20a as by a screw 87 extending through an opening in the plate and into the pole piece 558 of the field structure. As before, there is preferably clearance between the opening in the plate 20a and the screw 87 to permit adjustment of the field structure with respect to the plate. In this type of device a conventional spider 89, secured to the interior of the diaphragm 70 and to the pole piece 88 as by'a screw 90, may be employed.

Figs. 9 and 10 illustrate a loud speaker embodying another form of diaphragm made according to my invention. Here the diaphragm is provided with a supporting flange I01 and a flexible corrugated portion 102 corresponding to similar parts in the previously described modifications, but the body of the diaphragm is made up of two oppositely inclined frusto conical surfaces 103 and 104 connected by an annular surface 105 from which the voice coil supports 106 project; the supports being provided with the usual voice coils 107. In this modification the need for a separate central support or spider is eliminated, the diaphragm being provided with a central flexible corrugated portion 108 and a central plane portion 109 which is adapted to be secured to a suitable support. Thus the flexible corrugated portion 108, which is disposed Within the locus of the several voice coil supports, permits the annular body of the diaphragm to vibrate freely yet insures proper alignment ofthe diaphragm with respect to the remaining structure of the loud speaker.

Diaphragms of this type may be employed with loud speaker structures generally similar to those heretofore described and the same reference characters used in the description of the previous modifications are applied to corresponding part shown in Figs. 9 and 10 of the drawings. In this form, however, the supporting plate 20b differs from the plate 20 heretofore described in that no provision need be made for supporting the central portion of the diaphragm therefrom. The central portion of the diaphragm is supported adjacent the" front of the speaker inasmuch as the corrugated portion 108 and the central portion 109 lie in substantially the same plane as the edge or flange 101 and the corrugated portion 102. In order to support the central portion 109, I preferably employ a member such as the spoked member 111, the edge 112 or which is secured to the flange 46 as by screws 113 or any other suitable means; the central portion 114 being secured to the central portion 109 of the diaphragm as by a nut 115 and bolt 116 or any other convenient securing means. The central portion 114 is supported by spokes 117 which are preferably bowed outwardly, as shown, to eliminate any possibility of contact between vibrating portions of the diaphragm and the spokes.

Fig. 11 illustrates another form of diaphragm and speaker assembly in which the need for a separate supporting spider in the central portion of the diaphragm is eliminated. In this modification the dia phragm and the speaker structure in general are substantially identical with the construction shown in Figs. 1,

- 2 and 3, except for the central portion of the diaphragm and here again the same reference characters are employed.

to indicate parts which have not been substantially However, in this modification the diaphragm itself; indicated in general at 120, instead of continuing elite tlieapex of the cone as in Figs. 1, 2 and 3, is terminated short of the apex of the cone and has integrally formed with the body portion thereof a flexible corrugated portion 121 connecting the conical body of the diaphragm with the central plane portion 122. In the speaker the supporting plate 200 is formed with a depression 123 of greater depth than the depression 39 previously described and the central portion 1.22 of the diaphragm is secured to the center of the depression 123 by the bolt and nut 124-125 or by other convenient means. Otherwise, as noted above, the structure shown in this figure is substantially identical to that previously described'in connection with Figs. 1, 2 and 3.

In the modifications of Figs. 9, l and 11 the necessity for separate central supporting spiders is eliminated thus obviating the troubles which frequently occur with such devices "and cutting down the cost of the assembling operations while improving the accuracy of the support of the diaphragm itself. The diaphragms of Figs. 9 and '10 are particularly adapted to large installations of high output while the diaphragm shown in Fig. ll is of high efliciencyv and provides an economical assembly. Those skilled in the art will appreciate that the same principle of utilizing an integrally formed flexible portion adjacent the center of the diaphragm may be used with diaphragms of various different shapes and with arrangements of voice coils different from those illustrated herein.

A further modification of the invention is shown in Fig. 12 and in this instance a conical diaphragm is provided and it has a short voice coil support neck 151 thereon usually made as a unit therewith. A cylindrical voice coil 152 is supported on the neck 151 preferably by a voice coil support tube 153. A metallic cone housing 154 positions the diaphragm 150 and a top or outer pole, plate 155 is secured to the housing 154, which plate 155, has an annular opening 156 therein through which the voice coil support tube 153 extends in centered but spaced relation thereto. A magnet pot 157 is suitably secured tothe top plate. 155 and a magnet 158 is carried by the magnet pot 157 and extends through the tube 153 in centered but spaced relation thereto. As an important feature of this type of loud speaker of the invention, a corrugated spider 159 is suitably secured to the plate 155 and is fixed to the coil support neck 151 or the voice coil support tube to aid in resiliently centering the voice coil in the a ir gap intermediate the plate 155 and magnet 158.

From the foregoing description of preferred forms of my invention, it will be evident that I have provided diaphragms and loud speakers embodying the same giving advantageous results particularly in the fidelity of sound reproduction. The distribution of the forces throughout the diaphragms makes possible the application of more energy to the diaphragms without requiring that the weight of the diaphragms be increased in proportion. Thus more power can be applied to a diaphragm of a given size andweight constructed in accordance with my invention than can be utilized successfully by a conventional diaphragm of the same size and weight. The application of the forces by a plurality of voice coils spaced around the diaphragm is also advantageous in that the use of a, plurality of smaller voice coils makes more eiflieient of the metal necessary in constructing the fields and the voice coils than is possible in a single voice coil construction designed to have the same power. output. The reason for this is that design con-' side-rationsrequire the single voice coil and field structure to be of much larger diameter than the several smaller and voice coil structures used in loud speakers invention, and the smaller structures obvi- 31 g "makefm ore advantageous use of the-metal in the coils and in the field because more ampere turns can be obtained with the same quantity of wire in small coils than can be obtained in coils of larger diameter.

Because of the distribution of the vibratory forces imposed by the voice coils, diaphragms made according to my invention vibrate substantially uniformly throughout. Because of this more nearly uniform vibration, my diaphragms are more efficient size for size as sound reproducing devices than the conventional diaphragms,

in which the force is applied only at the apex of the diaphragm, the increase in efficiency being particularly marked in the larger sizes. Furthermore, with diaphragms embodying my invention, sufiicient input of power can be employed to render the natural periods of frequencies of vibration of the diaphragms inconsequential, and accordingly the diaphragms will vibrate more nearly in conformity to the voice currents flowing in the voice coils than has been possible with diaphragms of conventional type. The diaphragms are accurately supported in the speakers by flexible supporting portions at the periphery and also centrally, i. e., in the region within the locus of the voice coil supports. My loud speakers are compact and efficient and of relatively simple construction.

It is to be understood that various changes may be made in the preferred forms of my invention disclosed herein Without departing from the teachings of my invention. For example, the size, shape and contour of the diaphragms may be changed as may the number and spacing of the voice coil supports. The design of the field structures may be varied. The invention is useful in speakers of the permanent magnet type as well as in the dynamic type disclosed herein. Various other changes may be madewithin the precepts of the invention. It is therefore to be understood that my patent is not limited to the preferred forms described herein.

This application is a continuation-in-part of my copending application Ser. No. 615,719.

Having thus described my invention, what I claim is:

1. A loud speaker comprising a frame formed with an opening at one side, an acoustic diaphragm secured at its periphery to the frame around said opening and dished inwardly at its outer surface, a plurality of laterally spaced tubular voice .coil supports carried by and projecting from the inner surface of the diaphragm, said supports being symmetrically spaced with respect to the diaphragm with their axes parallel to the direc-.

tion of movement of the diaphragm and spaced from each other, an annular voice coil carried by each of the supports, a plurality of magnetic units carried by the frame and laterally spaced from each other to register with the voice coil supports respectively, each of the magnetic units including a central magnetic pole extending into the adjacent support and an annular magnetic pole lying around the adjacent support and defining with the central pole an annular air gap in which the annular voice coil carried by the support is movable.

2. The construction of claim 1 in which the supports, are spaced from the; center of the diaphragm and means are provided connecting the center of the diaphragm to the frame.

3. The construction of claim 1 in which the central portion of the diaphragm is connected to the frame through a flexible spider.

4. The construction of claim l in which the supports are spaced from the center .of the diaphragm and the central portion of the diaphragm is rigidly secured to the frame and is secured to the remainder of the diaphragm through a flexible corrugated portion.

5. The construction of claim 1 in which one of the supports is at the center of the diaphragm and the other supports lie in spaced annular array around said one c he. pp ts..-

1. A 9nd pe ker, amp i iasa hq s bra le d e- 9 h ng, a a! b ts? a, m ltipl ity f. at r ally spaced mgnetically actuated coils at laterally spaced FOREIGN PATENTS anartportions 0f the g sg and laterally spaced 312,756 Gre Bri i June 2 separate actuating magnetlc umts for the cells respec- 317,134 Great Britain 0 15, 1929 tively in magrtetically coupled relationship with the coils. 319,942 Great Britain Oct 3, 1929 5 Refe ggnces Cited in the file of this patent "UNITED STATES PATENTS 1,604,532 Kiegger Oct.26, 1926 2,034,882 Scribner Mar. 24, 1936 

